1. Field of the Invention
The present invention relates to a field of an optical communication which transmits information by use of a semiconductor laser or a field of an optical recording which executes recording and/or reproducing of information, and more particularly to a semiconductor laser having a stable wavelength and a large power.
2. Description of the Prior Art
A semiconductor laser has been performing an important role as a small-sized light source in the optical communication field or the optical recording field. And, recently, an output of the semiconductor laser is required to further increase more and more. Especially, in the case where the semiconductor laser is utilized as a light source of an optical disk capable of erasing its recorded memory, a high-power semiconductor laser which can focus to a diffraction limit becomes necessary.
Conventionally, in order to realize a power-up of the semiconductor laser, many approaches have been tried. These approaches are chiefly classified into three categories. First one is characterized by a method for improving a wave guide structure of a semiconductor laser having a usual single-transverse mode. Second one is characterized by a method for increasing light-emitting area by expanding a stripe width of an active layer; i.e. a method for manufacturing a so-called broad area type semiconductor laser. And, third one is characterized by a method for constituting an array of a plurality of lasers having modes being coupled with each other; i.e. a method for manufacturing a so-called phase lock type laser.
However, an oscillation output of a conventional single-stripe semiconductor is limited up to approximately several 10 mW, though it has a superior focusing characteristics. Furthermore, its wavelength is varied based on value change of injection current. And, in the case where this conventional single-stripe semiconductor is used in an optical disk system, an aberration is inherently generated due to wavelength dispersion in an optical lens. Therefore, it is earnestly desired to realize a semiconductor laser capable of suppressing wavelength fluctuation as less as possible.
On the other hand, a transverse-mode of the conventional broad area type semiconductor laser is a multiple mode. Therefore, this type semiconductor laser cannot be used in an optical disk system because it is impossible for this broad area type semiconductor laser to focus beam to a diffraction limit.
FIG. 7 shows a typical constitution of a phase lock type semiconductor laser. This phase lock type semiconductor laser includes a plurality of line-shaped active layers 1a, 1b, - - - , 1j having wave guide structures disposed in parallel with each other. And, these active layers 1a, 1b, - - - , 1j are constituted in an array wherein respective wave guide modes are optically coupled with each other.
If all the phases A of beams emitted from end surfaces of respective active layers 1a, 1b, - - - , 1j are uniformly locked, an output B can be highly enhanced. To the contrary, if all the phases A of beams emitted from the end surfaces of respective active layers 1a, 1b, - - - , 1j are not locked, the output B is weakened due to interference.
It is practically impossible to realize a 0-order coupling in which all the phases A are perfectly coherent. Especially, if an array number is increased to obtain a higher output, a realization of the 0-order coupling becomes further difficult.
Moreover, even if the 0-order coupling can be realized under a limited condition, this 0-order coupling is easily deteriorated since it is influenced by change of injection current value and fluctuation of circumferential temperature. For example, a propagation constant of each wave guide is varied by the change of injection current value and the fluctuation of circumferential temperature. Therefore, it is impossible to focus beams to the diffraction limit.
Accordingly, a semiconductor laser having a property suitable for a light source of the optical disk has not been realized yet.